The traditional prior art relay driver consists of a single transistor with the relay load connected to the collector. The base drive current is tailored to a particular relay as a function of its maximum operate current and the minimum transistor Beta. However, if such a driver is required to be flexible and function with relays which have different operate currents, the base drive must be calculated for the relay having the largest operate current. Therefore, power is wasted when the driver is used with relays having smaller operate currents. For example, assuming a minimum Beta of 30, if one relay requires 250 mA to operate the base drive must be 8.3 mA, of which over 8 mA is wasted to drive a 5 mA relay.
One solution to this problem is to increase the effective Beta of the transistor driver by using a Darlington configuration. If the effective Beta can be increased to 500, then only 0.5 mA of base drive is necessary for either relay in the previous example. This minimizes the quiescent current drain, but because of the Darlington stage the voltage available to drive the relay will be additionally reduced by a V.sub.be. In certain applications this may be acceptable, but with a low voltage relay supply (e.g., 5 V) an additional reduction of a V.sub.be is such a significant degradation that a 5 volt relay may not operate under worst case conditions. Also, if the relay winding could be redesigned to operate at this lower voltage it would necessitate a higher relay operate current, and thus a further increase in power.